In industrial chemical reactors, complicated processes including flow, mixing, dispersion, transfer and reaction of several different phases are always involved. Size, concentration, and velocity distributions of bubbles, droplets or solid particles are the most direct and effective indexes reflecting the internal characteristics of reactors and they are also the key parameters for the analysis, selection, design and scale-up of multiphase reactors.
At present, various probe methods are most commonly used based on optical fiber reflex or conductance (capacitance). Both methods have been applied successfully in the study of gas-liquid and liquid-solid systems, for example, the monofilament capacitance probe measurement system of phase containing rate and phase interface in multiphase pipe flow (CN 1865966A) and the measurement method and device of the two-phase flow parameter based on the double capacitance probe (CN 101413911A). However, both of them have some limits. Multiphase flow to be measured should be conductive for the conductance probe, while the fiber optic probe is very easy to be broken. Signals will interfere with each other when there are two or more dispersed phases. In addition, there are ultrasonic tomography, gamma ray tomography, electronic tomography technology, etc. The major advantage is non-intrusive, but they cost too much and their resolutions of time and space still need to be improved.
Optical photography including non-invasive and invasive photography is the most intuitive measuring method. The outstanding advantage of non-intrusive photography is that it does not interfere with the flow field. And its shortcomings are that the measured reactors must be transparent or installed with viewports, and the holdup of bubbles or solid particles must be lower. Invasive photography such as optical fiber endoscope can be directly inserted into the reactor for online measurement. But the measurement accuracy is limited by the fiber diameter. At the same time, the wide-angle lens installed at the front end of the endoscope will cause a larger image distortion.
The prominent advantage of a telecentric lens is that the image is almost no distortion (for example, its distortion coefficient is less than 0.1% compared to 1-2% of a regular wide-angle lens). An image measuring device (CN 203405417U) in two-phase flow in a pipeline was disclosed, in which the droplets to be measured are illuminated by a lighting source through a transparent pipe or a viewport. Surface of the liquid film is illuminated by a laser source, and images of drops or liquid film of the area to be measured are obtained by a telecentric lens and a CCD or a CMOS image sensor. The droplet image out of the lens depth of field by comparing defocus radius of a single droplet image with allowable dispersion circle size of measurement system is removed; particle size, velocity and movement direction from the particle images within the depth of field are extracted, and the droplet size and velocity distribution, and the average concentration and average volume flow rate during statistical time are obtained by statistics on a large number of droplets within the depth of field in the measured region; the film thickness based on the distance between surface and wall in film image are determined.
Although the device has solved the problem of image distortion, it has the shortcomings of non-intrusive optical photography mentioned above and strict requirements on reactors to be measured.
Invasive telecentric imaging may solve the shortcomings existing in the above technology, but some technical problems still need to be overcome: (1) invasive telecentric probe should be long enough to measure anywhere of the multiphase reactors; (2) light intensity will reduce when propagated through fluid medium in the multiphase reactor, and adequate illumination is necessary to make sure the exposure enough in the process of photographing; and (3) it is necessary to clearly capture the image of the high-speed moving particles, as the fluid in the multiphase reactor is in high speed. In addition, the probe size should be as small as possible not to disturb the flow field.